In our co-pending U.S. patent application Ser. No. 839,548, for Digital Data Communications Adapter, filed on even date herewith, there is described a bit oriented multipoint data communication system, which may be advantageously employed for the serial communication of data between processing terminals. As is described therein and as is shown in FIG. 1, each frame of serially transmitted data begins and ends with a unique flag byte, employed by both the transmitter and the receiver terminal for frame synchronization purposes. Since the flag byte is to appear in the data frame only where intended, usually at the beginning and end of the frame, it is extremely important that some means be provided to prevent the flag code from appearing within the address, control, information (I), and frame check sequence (FCS) fields; otherwise the frame would be prematurely terminated.